The present invention relates to an internal combustion engine operating with a homogeneous charge of combined gaseous and liquid fuel ignited by compression. The present invention further relates to such an engine where exhaust gas is used with the combustible mixture and where gaseous fuel may be used to inject liquid fuel.
Current production engines include diesel engines; engines with a homogeneous charge ignited at a particular location as by a spark; and lean burn, spark ignited engines where a locally rich mixture is ignited and provides ignition of an overall lean mixture. These engines are all limited in the maximum efficiency that can be achieved at a given level of emission of nitrogen oxides (NOx) because of the high temperature that is produced in the zone where combustion occurs. In these engines, efficiency is improved by increasing the mean combustion temperature, but this pushes the temperature in the combustion zone into the range in which NOx is generated. This is the case in both the diffusion flame front of a diesel, where the rate of mixing determines the rate of combustion, and in a propagating flame front where the flame travels through a homogeneous or lean charge from a point of ignition. The operation of the above engines will be briefly described as background for Homogenous Charge Compression Ignition (HCCI) engines including HCCI engines embodying the present invention.
More particularly and in diesel engines, only air, with a greater of less admixture of exhaust gas, is compressed in the cylinder, and combustion of a liquid fuel is initiated by heat of compression when fuel is first injected. This combustion continues as the rest of the fuel is injected and oxygen from the air mixes therewith. Conceptually, the same amount of gas is always compressed and, when greater power is required, more fuel is added by injecting fuel over a longer period of time. The efficiency of diesel engines is relatively high because of the high compression ratio, particularly at less than full power where the overall fuel to air ratio is very lean and because the flow of intake gas to the cylinder is not restricted.
It is apparent that, in diesel engines, the fuel should ignite as soon as injection begins; and the cetane number, by which diesel fuels are rated, is a measure of the ease of ignition, a higher cetane number indicating greater ease of ignition.
In the homogeneous charge and lean burn engines, evaporated liquid or gaseous fuel is mixed in an approximately stoichiometric ratio with air before or shortly after the air is provided to the cylinder; the mixture is compressed; and combustion is initiated by ignition, typically by an electric spark, at a point from which the flame front propagates. It is evident that in these engines the fuel should not ignite by compression until the spark occurs so that the fuels used in these engines are relatively difficult to ignite and thus would have a low cetane number. In conventional homogeneous charge engines, the power is controlled by restricting the amount of air provided to the cylinder and varying the amount of fuel provided correspondingly.
It is known to provide a dual fuel variation of the homogeneous charge engine, for example as described in U.S. Pat. No. 3,144,857, which uses an approximately stoichiometric mixture of air and a gaseous primary fuel that can be highly compressed without ignition. This mixture is then ignited at a point, from which the flame front propagates, by a pilot injection of a relatively small amount of diesel fuel that is ignited by compression of the mixture.
It is apparent that, in all of these types of engines, the ignition and start of combustion can be controlled by selecting the point, as an angular position of a crankshaft, where the spark or fuel injection occurs. It is known in some engines of all of these types to employ Exhaust Gas Recirculation (EGR), in which a portion of the gas exhausted from the cylinder is mixed with air being provided to the cylinder. This is typically done to reduce emissions, and complex control arrangements are known for selecting the portion of exhaust gas recirculated in relation to engine load, speed, temperature, amount of oxygen in the exhaust, and the like; these arrangements also selecting related engine control factors such as the time and amount of fuel injected and the time of ignition.
In contrast to the above-described engines, Homogenous Charge Compression Ignition (HCCI) engines produce both very low emissions and diesel engine efficiencies by avoiding the high thermal gradients that are present in either a diffusion or a propagating flame front. In present HCCI engines, an evaporated liquid or a gaseous fuel is mixed with air before or shortly after the air is provided to the cylinder to form a homogenous charge; the mixture ignites as compression continues; and combustion occurs without a defined flame front since combustion is initiated throughout the combustion chamber and the rate of heat release is determined by the chemical reaction rate of the charge.
In HCCI engines, a charge having the same amount of air, with a greater of less admixture of exhaust gas, is always compressed in the cylinder and power is controlled by varying the amount of fuel added to and homogeneously mixed with the charge, with ignition and combustion of the lean mixtures being achieved by high compression ratio and recirculation of hot exhaust gas.
The rapid and homogenous burning possible with HCCI engines has, in concept, the potential to reduce exhaust emissions, retain diesel-like efficiency, and reduce engine cost, particularly in high speed engines. HCCI engines can greatly reduce the level of NOx emissions at a given efficiency due to the more uniform in-cylinder temperature distribution; and, because no flame front is present, the mean cylinder temperature of a HCCI engine is equal to that of a diesel engine without the peak temperature entering the NOx generation region as in the diesel and other engines described above.
HCCI engines should be less expensive to produce than diesel engines or lean burn spark ignited engines since HCCI engines do not necessarily require the high pressure oil system, including pump, distribution system, and injectors of a diesel and do not require the ignition system of a spark ignited engine with spark plugs, spark plug wires, and ignition coils, which offer serious durability problems on heavy duty engines.
Heretofore however, HCCI engines have not been practical because with the previously employed fuelsxe2x80x94diesel or natural gasxe2x80x94and arrangements for using these fuels, HCCI engines operate over a limited range of loads and speeds and provide a very low power densityxe2x80x94that is, produce a very low amount of power for the displacement and speed of such an engine. These limitations of HCCI engines have resulted from difficulties in controlling the ignition timing of the engine and in producing a long enough combustion event to prevent high peak cylinder pressures.
For the purposes of the present application, the terms xe2x80x9cignition timingxe2x80x9d and xe2x80x9cignition delayxe2x80x9d are related. Typically, ignition timing refers to the start of desired combustion in relation to a particular angle of crankshaft rotation; and ignition delay refers to the elapsed time before the start of combustion after such an angle, completion of injection of a fuel, or the like. Terms such as xe2x80x9cburn durationxe2x80x9d, xe2x80x9cduration of combustionxe2x80x9d, and length of xe2x80x9ccombustion eventxe2x80x9d refer to the elapsed time for combustion to be completed after the start of combustion, this elapsed time being measured absolutely or as an angle of crankshaft rotation.
The previously utilized method for selecting the ignition timing of a HCCI engine is by control of Exhaust Gas Recirculation (EGR). That is, the proportion of hot exhaust gas recirculated is increased and the proportion of air is reduced so that the cylinder temperature during compression is increased and ignition delay is shortened. However, the operating speed range of HCCI engines with such EGR control is inherently limited because increasing the EGR to shorten ignition delay lengthens the duration of the following combustion due to the lesser availability of oxygen.
As a result, at low speeds where longer ignition delays are acceptable because there is longer time between fuel injection and first indication of combustion, little EGR is needed to promote ignition, while high EGR levels are needed to lengthen the combustion once it has begun. Conversely, at higher speeds, where high levels of EGR are needed to accelerate the ignition process, these high levels lengthen the combustion period and so limit the maximum operating speed of the engine.
Diesel fueled HCCI engines have a very narrow range of operation and low levels of specific power output because diesel fuel has a short ignition delay, due to its very low autoignition temperature, which makes compression ignition very easy to achieve. However, this ease of ignition limits the range of fuel to air ratios to the very lean, and therefore slow to ignite, mixtures required to lengthen ignition delay to levels acceptable in HCCI engines. This limited range of mixtures and ignition delay limits the range of operation, and the lean mixtures limit the energy released and give low specific power output.
Even with these lean mixtures, however, diesel fuel burns rapidly once ignited; and this results in rapid pressure rise which is limited by the mechanically allowable cylinder pressure. As a result, boost pressure, such as provided by a turbocharger, must be restricted; and this further contributes to relatively low specific power output.
It is known, as in U.S. Pat. No. 5,832,880, to operate a diesel fueled HCCI engine with sufficient EGR to always ensure ignition and then inject water to delay the start of combustion, as determined by a sensor, to a desired time. It is apparent that the water makes combustion more difficult and, in effect, lowers the cetane number of the diesel fuel. As a result, the delay of combustion would be accompanied by longer burn duration so that and ignition delay and burn duration are not independently controlled.
Natural gas fueled HCCI engines have a very narrow range of operation and very low levels of specific power output because, while natural gas burns more slowly than diesel fuel and thus provides a more extended heat release duration and slower pressure rise, natural gas is more difficult to autoignite than diesel fuel so that very high levels of EGR must be used to produce compression temperatures at which natural gas will compression ignite in a reasonable period of time. These high levels of EGR limit the energy released, giving low specific power output. Also, these EGR levels lengthen the combustion duration that reduces the maximum speed at which the engine can be operated and thus limits both the specific power output and the operating range.
The present invention is a Homogenous Charge Compression Ignition (HCCI) engine which provides the above-described advantages of low emissions and high efficiency together with high power density, a wide range of operation, and reduced cost.
One aspect of the present invention is effective with an HCCI engine in which the primary fuel, typically greater than 95%, is a gas with a relatively slow burn rate, typically natural gas, and in which ignition timing is controlled by the addition thereto of varying amounts of a high cetane number fuel, typically diesel fuel, before or early in the compression stroke. The amount of high cetane fuel that is added depends on engine speed and load, and is selected to insure that the initiation of the combustion event is phased properly with crankshaft position.
The ignition timing is thus controlled substantially independently of the burn duration, which, in another aspect of the invention, is controlled substantially independently of the burn delay by varying the amount of Exhaust Gas Recirculation (EGR) which may be varied in conjunction with the fuel to air ratio.
In yet another aspect of the present invention, which independently controls the ignition delay and burn duration as before stated, the load and speed of a subject HCCI engine may be selected over a wide range by varying either the overall fuel to air ratio or the boost pressure or by varying these engine operating conditions together in any suitable manner.
In a further aspect of the present invention, as when it is utilized with a high cetane fuel such as diesel fuel or lubricating oil which is liquid and of low volatility, the gas and liquid fuel are injected together and early in the compression stroke when the cylinder pressure is relatively low. The gas injection atomizes the liquid without the need for the complex, high-pressure components required by a diesel engine and also provides increased turbulence during combustion.
More specifically in the present invention, the ignition delay is controlled by varying the cetane number of the overall fuel mixture, this number being higher for a more easily ignited fuel as before mentioned, by the addition of varying amounts of high cetane fuel to a primary low cetane fuel as by the above-described injection of varying amounts of the high cetane liquid fuel into the primary, gaseous low cetane fuel.
This control is effective because the cetane number of a fuel mixture can be varied by combining different ratios of two fuels; and, if a small amount of diesel fuel or other high cetane number fuel is combined with natural gas, the ignition delay of the mixture will vary in a predictable manner with the amount of cetane improver added. The ignition timing of a HCCI engine can thus be selected independently of either the amount of EGR or the overall fuel to air ratio. Further, when a controlled amount of the high cetane fuel is injected every cycle, this independent ignition timing of the HCCI engine can be varied on a cycle by cycle basis.
The use, in an HCCI engine and in accordance with the present invention, of varying amounts of high cetane fuel, such as diesel fuel, in combination with a low cetane fuel, such as natural gas as a primary fuel, offers at least four advantages over other HCCI engine arrangements and methods of operation. First and as just stated, the crank angle at which ignition occurs can be controlled on a cycle to cycle basis. Second and because ignition delay is controlled independently from the amount of EGR, varying levels of EGR can be used to maintain a near constant crank angle burn duration over a wide range of engine speeds. Third and because the cylinder temperature is largely uniform and the fuel to air ratio is not used to control ignition delay and is not necessarily used to control burn duration, the load range of the engine can be extended by operation over a range of lean mixtures. Fourth and because of the relatively slow burn rate of the low cetane fuel in comparison with the high cetane fuel, the power density of the engine is increased because lower EGR levels and higher boost levels are practical.
Further, an HCCI engine embodying the present invention is advantageous in relation to a lean burn, spark ignited engine because the lean limit is lower in the subject engine due to the added high cetane fuel contributing energy to combustion and because the lack of a propagating flame front significantly reduces burn duration. The fuel economy of the subject engine will thus be greater while the power density is increased and the engine cost is lowered by the absence of an ignition system.